Certain optronic systems emit electromagnetic pulse trains in the direction of a target of the scene in order to designate or identify it. These electromagnetic pulses are preferably light pulses, for example emitted from flash lamps or lasers.
Others optronic systems perform the function of acquiring images or sequences of images of the scene with variable exposure times. Certain systems conduct scans of a line of sight in the scene or trigger shuttering devices. Others perform measurements for location of the target or measurements of angular separation (angular distances) and positions between the direction of a target on the scene (such as a tank, an airplane) and the direction of a vehicle or device (such as a missile) for example, or by measurements of orientation of the line of sight and distance between the target and the optronic system.
Certain components of these optronic systems are thus controlled by a law of temporal behaviour requiring precise specifications, for example, of the order of the nanosecond for laser pulses and up to the millisecond for cameras.
The difficulty is in synchronizing all of these operations without exposing the optronic systems operating on the scene.
In particular, in order to improve their mode of operation, their scope or their accuracy, it is useful to ensure that these independent optronic systems operate in a synchronous fashion.
This is the case for example of optronic semi-active laser systems having on the one hand, a laser transmission system operating at a certain rate, and on the other hand, data acquisition systems such as a distance gauging or imaging system looking to acquire the image of the laser illumination spot designating or identifying the target in the scene.
Another example is the case of independent mobile cameras seeking to be synchronized so as to improve for example the 3D rendering of a same given scene comprising of mobile units, for example, objects or targets that are moving in the scene, these camera also being capable of being mounted on independent platforms that are themselves mobile.
This is also the case for an imager that it is desirable to protect from a friendly pulsed laser jammer by carrying out a check of the exposure time for acquisition of images between the emission of successive pulses emitted by the jammer.
It is known that the synchronization of these operations is performed by outputting a signal via a wired connection or a microwave radio link for synchronization of one of the optronic systems with the other optronic systems to be synchronized if possible. However, the wired solution requires collocation of the optronic systems that are to be operating together and distributing the synchronization signal to each other.
In fact, if the optronic systems are not collocated, they must transmit to each other by microwave radio beam the synchronization signals. In this case, a synchronization tolerance is necessary, taking into account the transmission delay of the radio link. For example, for a distance of 300 m, there is a delay of 1 μs in view of the propagation at the speed of light. In order to reduce this tolerance, it is necessary to use a method for determining this transmission delay so as to compensate for it.
Another possibility is to incorporate a sensor in the optronic systems to be synchronized, that is capable of acquiring an optical signal, for example a laser pulse train emitted by one of the optronic systems in order to carry out the synchronization over at least one of the laser pulses. However, this requires the transmission of an optical signal by one of the optronic systems that may be detectable by enemy optronic systems.
Moreover, optronic systems designed to detect laser illumination pulses must have a long period of continuous exposure so as to perform the detection with the disadvantage of reducing the contrast between the spot of the laser pulse and the scene and thus the scope of the optronic systems.
Thus, it is not possible to synchronize passive independent equipment systems without transmitting, between the various optronic systems operating on one and the same scene, a microwave radio signal emitted by one of the optronic systems, or to synchronize active or semi-active independent equipment systems from the first laser pulse emitted by one of the optronic systems.